1. Field of the Invention
The present invention relates to a wet multi-plate clutch used in an automatic transmission. More particularly, the present invention relates to an improvement in an urging arrangement for friction engagement elements.
2. Related Background Art
A wet multi-plate clutch has mainly been used in an automatic transmission since a torque transmitting property of such a clutch from a clutch releasing condition to a clutch lockup (frictional engagement) condition is very smooth. Further, in order to damp transmission shock of the automatic transmission, there has frequently been used a technique in which an elastic member is interposed between a piston and a friction engagement element.
FIG. 10 is an axial sectional view showing an example of a conventional piston. An elastic member, i.e., a wave spring 40 for damping transmission shock is disposed between urging surfaces 51 and 52 of a piston 50. Although the wave spring is arranged so that it is embedded into the urging surfaces 51, 52, the reason why such an arrangement is adopted is that an axial dimension of the wet multi-plate clutch is reduced.
However, in the wet multi-plate clutch having the above-mentioned arrangement, since the urging surfaces 51, 52 are flush with each other, when the piston urges the friction engagement elements, surface pressure distribution of a lockup load is widened, so that there arises a phenomenon (referred to as xe2x80x9cshudder phenomenonxe2x80x9d hereinafter) in which transmitting torque is varied repeatedly in friction engagement.
Accordingly, an object of the present invention is to provide a wet multi-plate clutch that can suppress a shudder phenomenon, while maintaining a reduced axial dimension of the wet multi-plate clutch.
To achieve the above object, the present invention provides a wet multi-plate clutch comprising a clutch case and hub disposed on a same axis, a first friction engagement element disposed in the clutch case, a second friction engagement element disposed on the hub, and a piston for applying an axial load to the first and second friction engagement elements, and wherein a recessed groove is formed in a surface of the piston opposing to the first and second friction engagement elements, and a wave spring is disposed within the recessed groove, and only a portion of the opposed surface at an outer diameter side or an inner diameter side of the recessed groove urges the respective friction engagement elements.
Further, in the present invention, the contact portion of the piston for urging the friction engagement element may be substantially line-shaped. Further, in the present invention, the urging surface of the piston may be arc-shaped. Here, the xe2x80x9curging surfacexe2x80x9d means a surface provided on the piston and adopted to contact with the friction engagement element when a lockup load is applied to the friction engagement elements.
In a preferred embodiment of the present invention, a separator plate is used as the first friction engagement element and a friction plate is used as the second friction engagement element. Further, a fundamental construction of the piston is substantially the same as that of the above-mentioned conventional piston. In addition, the separator plates are installed within the clutch case through splines for axial movement, and the friction plates are installed on the hub through splines for axial movement.